In a so-called videoscope mounted with solid-state image pickup elements at the tip of the insertion portion of an endoscope, a system for outputting a high precision video signal of HDTV specification and/or PC format has been commercialized. It is easy to realize a videoscope of standard specifications with pan focus if it has NTSC/PAL resolution. However, it becomes difficult to obtain a proper balance of optical specifications with pan focus due to effects of lowering sensitivity caused by the reduction of pixels with solid-state image pickup elements and deterioration of contrast caused by diffraction and other factors. Additionally, optical specifications when adopting progressive read-out type, primary color, single-plate image pickup units or multi-plate image pickup units, such as two-plate or three-plate units, which provide a high precision image other than by increasing the number of pixels, are not established with pan focusing.
Therefore, in an objective optical system that provides high precision images, a decrease of f-number that is necessary to ensure a sufficient image brightness for viewing a high precision image results in an insufficient depth of field that must be compensated for by including a focusing function in the objective optical system.
The most general construction for providing the focusing function in an endoscope is a mount of the lens moving type. For a driving system in which lenses are moved by such a mount, a mount that can be arranged at the tip of the endoscope in the vicinity of a moving lens is desirable from the viewpoint of power transmission efficiency and positional accuracy.
Objective optical systems described in the following cited references have been known as prior art examples of objective optical systems for an endoscope having a focusing function by lens movement: Japanese Patent S55-15004; Japanese Patent H4-3851; Japanese Laid-Open Patent Application H7-181377; Japanese Patent S55-15005; Japanese Laid-Open Patent Application H11-316339; Japanese Laid-Open Patent Application 2000-267002; Japanese Laid-Open Patent Application 2000-330015; Japanese Laid-Open Patent Application 2002-28126; and Japanese Laid-Open Patent Application 2002-357773.
In order to apply the optical systems described in these patent references to an endoscope with a focusing function having high-precision solid-state image pickup elements, various problems must be addressed that will be discussed in the following comments.
However, first, definitions of the terms “lens element” and “lens component,” that relate to the above cited patent references, as well as to the detailed description of the present invention that will follow later, will be given. The term “lens element” is herein defined as a single transparent mass of refractive material having two opposed refracting surfaces, which surfaces are positioned at least generally transverse to the optical axis of the lens. The term “lens component” is herein defined as (a) a single lens element spaced so far from any adjacent lens element that the spacing cannot be neglected in computing the optical image forming properties of the lens elements or (b) two or more lens elements that have their adjacent lens surfaces either in full overall contact or overall so close together that the spacings between adjacent lens surfaces of the different lens elements are so small that the spacings can be neglected in computing the optical image forming properties of the two or more lens elements. Thus, some lens elements may also be lens components. Therefore, the terms “lens element” and “lens component” are not mutually exclusive terms. In fact, the terms may frequently be used to describe a single lens element in accordance with part (a) above of the definition of a “lens component.” Alternatively, a lens component may frequently be made by cementing lens elements together. Neither a filter with two flat faces nor a cover glass is a lens component.
In the above cited patent references, Japanese Patent S55-15004, Japanese Patent H4-3851, and Japanese Laid-Open Patent Application H7-181377 relate to objective lenses for endoscopes, and in particular to embodiments where a moving lens group is composed of plural lens components that are held in a frame. It is unavoidable that a moving lens group with a frame becomes long and heavy when the moving group is composed of plural lens components. This is undesirable because the torque of an actuator for moving the moving lens group increases and the actuator becomes large in size. Embodiments described in the above cited Japanese Laid-Open Patent Application H11-316339 having a moving lens group of positive refractive power are also undesirable for the same reasons described above with regard to Japanese Patent S55-15004, Japanese Patent H4-3851, and Japanese Laid-Open Patent Application H7-181377.
An embodiment of an objective lens for an endoscope described in the above cited Japanese Patent S55-15005 is constructed for moving only one lens component, but the ray height on the first lens component is so high in this construction that the field of view angle cannot be widened.
Embodiments having a moving lens group of negative refractive power have been described in the above cited Japanese Laid-Open Patent Application H11-316339 and Japanese Laid-Open Patent Application 2000-267002. In these embodiments, the entire length of the optical systems is long and the length of the rigid tip of the endoscope increases so that in endoscopes using such optical systems and that are capable of adjustment to different viewing angles, operability related to such adjustments deteriorates. Additionally, these optical systems are undesirable as optical systems for endoscopes strictly in terms of focusing considerations because the fluctuations of focal length associated with the lens movement are too large.
In embodiments of objective lenses for endoscopes described in the above cited Japanese Laid-Open Patent Application 2000-330015 and embodiments of an endoscope device described in the above cited Japanese Laid-Open Patent Application 2002-28126, the moving lens component is on the image most side, and these embodiments are undesirable because of the large outside diameter of the lenses required, and the large size in the radial direction of any frame mounting such a large size lens that is used with any mechanism for moving such a lens.
An optical system for an endoscope has also been described in the above cited Japanese Laid-Open Patent Application 2002-357773 with the lens component on the image most side having negative refractive power. This optical system is undesirable for use in combination with primary color single plate image pickup elements or multiplate image pickup units, such as two-plate or three-plate units, because the exit pupil distance is generally shortened.